U.S. patent application number 15/973338 was filed with the patent office on 2019-08-01 for polyimide film for graphite sheet, graphite sheet prepared by using the same and method for preparing graphite sheet.
The applicant listed for this patent is SKCKOLONPI Inc.. Invention is credited to Sung Il CHO, Jeong Yeul CHOI, Kyung Su KIM, Dong Young WON.
Application Number | 20190233293 15/973338 |
Document ID | / |
Family ID | 63078193 |
Filed Date | 2019-08-01 |
![](/patent/app/20190233293/US20190233293A1-20190801-D00001.png)
![](/patent/app/20190233293/US20190233293A1-20190801-D00002.png)
United States Patent
Application |
20190233293 |
Kind Code |
A1 |
WON; Dong Young ; et
al. |
August 1, 2019 |
POLYIMIDE FILM FOR GRAPHITE SHEET, GRAPHITE SHEET PREPARED BY USING
THE SAME AND METHOD FOR PREPARING GRAPHITE SHEET
Abstract
The present disclosure provides a polyimide film prepared from a
precursor composition containing a polyamic acid and an organic
solvent and having a value of (first FWHM-second FWHM)/(first
FWHM+second FWHM) which is less than 0.4, a graphite sheet prepared
from the polyimide film, and a method for preparing a graphite
sheet.
Inventors: |
WON; Dong Young; (Seoul,
KR) ; KIM; Kyung Su; (Seoul, KR) ; CHO; Sung
Il; (Yongin-si, KR) ; CHOI; Jeong Yeul;
(Anyang-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SKCKOLONPI Inc. |
Chungcheongbuk-do |
|
KR |
|
|
Family ID: |
63078193 |
Appl. No.: |
15/973338 |
Filed: |
May 7, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01P 2006/32 20130101;
C08J 2379/08 20130101; C01B 32/205 20170801; C08J 5/18 20130101;
C09K 5/14 20130101; C08G 73/1071 20130101; C08G 73/1003
20130101 |
International
Class: |
C01B 32/205 20060101
C01B032/205; C08G 73/10 20060101 C08G073/10; C08J 5/18 20060101
C08J005/18; C09K 5/14 20060101 C09K005/14 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 30, 2018 |
KR |
10-2018-0011176 |
Claims
1. A polyimide film prepared from a precursor composition
containing a polyamic acid and an organic solvent and having a
value of the following Formula 1 less than 0.4: (first FWHM-second
FWHM)/(first FWHM+second FWHM) Formula 1 wherein, when a
diffraction peak of (010) plane according to XRD analysis of the
polyimide film is in the form of a single peak, the first FWHM(full
width at half maximum) indicates a FWHM of the single peak; and
when the diffraction peak of (010) plane is in the form of a
plurality of single peaks or in the form of a plurality of single
peaks overlapped with each other, the first FWHM indicates an
average of individual FWHMs of the single peaks, and when a
diffraction peak of (102) plane according to XRD analysis of the
polyimide film is in the form of a single peak, the second FWHM
indicates a FWHM of the single peak; and when the diffraction peak
of (102) plane is in the form of a plurality of single peaks or in
the form of a plurality of single peaks overlapped with each other,
the second FWHM indicates an average of individual FWHMs of the
single peaks, wherein the precursor composition satisfies the
following conditions (a) to (c): (a) a viscosity of the precursor
composition is from 55,000 cP to 900,000 cP; (b) a heat treatment
temperature for heating and drying the precursor composition after
casting the precursor composition is from 150 degrees C. to 200
degrees C.; and (c) a film intermediate derived from the precursor
composition is stretched in at least one of a machine
direction.
2. (canceled)
3. The polyimide film of claim 1, wherein the polyamic acid is
contained in an amount 15 to 20% by weight based on the total
weight of the precursor composition.
4. The polyimide film of claim 1, wherein the film intermediate is
stretched so that the polyimide film has a thickness of 20 to 125
micrometers.
5. The polyimide film of claim 1, wherein a stretching ratio in the
at least one of the machine direction and the transverse direction
is +3% or more and +25% or less.
6. The polyimide film of claim 1, wherein the polyamic acid is
prepared by polymerization of dianhydride monomers and diamine
monomers.
7. The polyimide film of claim 1, wherein the first FWHM is 35
degrees or more and 80 degrees or less, and the second FWHM is 43%
or more and 92% or less of the first FWHM.
8. The polyimide film of claim 1, wherein the value of Formula 1 is
0.01 or more and 0.37 or less.
9. A graphite sheet derived from the polyimide film according to
claim 1 and having a thermal conductivity of 1400 W/mK or more.
10. A method for preparing a graphite sheet, comprising: preparing
a polyimide film having a value of the following Formula 1 less
than 0.4; carbonizing the polyimide film; and obtaining a graphite
sheet by graphitizing the carbonized polyimide film, wherein the
graphite sheet has a thermal conductivity of 1400 W/mK or more:
(first FWHM-second FWHM)/(first FWHM+second FWHM) Formula 1
wherein, when a diffraction peak of (010) plane according to XRD
analysis of the polyimide film is in the form of a single peak, the
first FWHM(full width at half maximum) indicates a FWHM of the
single peak; and when the diffraction peak of (010) plane is in the
form of a plurality of single peaks or in the form of a plurality
of single peaks overlapped with each other, the first FWHM
indicates an average of individual FWHMs of the single peaks, and
when a diffraction peak of (102) plane according to XRD analysis of
the polyimide film is in the form of a single peak, the second FWHM
indicates a FWHM of the single peak; and when the diffraction peak
of (102) plane is in the form of a plurality of single peaks or in
the form of a plurality of single peaks overlapped with each other,
the second FWHM indicates an average of individual FWHMs of the
single peaks, wherein the preparation of a polyimide film satisfies
the following process conditions (a) to (c): (a) a viscosity of a
precursor composition containing a polyamic acid and an organic
solvent is from 55,000 cP to 900,000 cP; (b) a heat treatment for
heating and drying the precursor composition after casting the
precursor composition is conducted at a temperature of from 150
degrees C. to 200 degrees C.; and (c) a film intermediate prepared
by the heat treatment is stretched at least once to obtain the
polyimide film.
11. (canceled)
12. The method for preparing a graphite sheet of claim 10, wherein
the preparation of a polyimide film includes: preparing a precursor
composition under the process condition (a); forming a film
intermediate by heat-treating the precursor composition under the
process condition (b); and stretching the film intermediate in at
least one of a machine direction and a transverse direction under
the process condition (c).
13. The method for preparing a graphite sheet of claim 12, wherein
the film intermediate is stretched so that the polyimide film has a
thickness of 20 to 125 micrometers.
14. The method for preparing a graphite sheet of claim 13, wherein
the film intermediate is stretched at a temperature of 20 degrees
C. to 40 degrees C.
15. The method for preparing a graphite sheet of claim 12, wherein
the film intermediate is stretched at a stretching ratio of +3% or
more and +25% or less in at least one of the machine direction and
the transverse direction.
16. The method for preparing a graphite sheet of claim 12, wherein
the precursor composition is prepared by mixing the polyamic acid
in an amount of 15 to 20% by weight and the organic solvent in an
amount of 80 to 85% by weight based on the total weight of the
precursor composition.
17. The method for preparing a graphite sheet of claim 12, wherein
the polyamic acid is prepared by polymerization of dianhydride
monomers and diamine monomers.
18. The method for preparing a graphite sheet of claim 10, wherein
the first FWHM is 35 degrees or more and 80 degrees or less, and
the second FWHM is 43% or more and 92% or less of the first
FWHM.
19. The method for preparing a graphite sheet of claim 10, wherein
the value of Formula 1 is 0.01 or more and 0.37 or less.
20. The method for preparing a graphite sheet of claim 10, wherein
the polyimide film is carbonized by heat-treating the polyimide
film at a temperature of 1,000 degrees C. to 1,500 degrees C.
21. The method for preparing a graphite sheet of claim 10, wherein
the carbonized polyimide film is graphitized by heat-treating the
carbonized polyimide film at a temperature of 2,500 degrees C. to
3,000 degrees C.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a polyimide film for a
graphite sheet, a graphite sheet prepared using the same, and
method for preparing a graphite sheet.
BACKGROUND
[0002] Recently, electronic devices are gradually becoming lighter,
smaller, thinner and highly integrated in its structure. As a
result, many problems attributable to a heat load have been caused
due to the increase in heat generation amount per unit volume.
Representative problems include, for example, problems directly
affecting the performance of an electronic device, such as a
decrease in operation speed of a semiconductor due to a heat load
of an electronic device, a shortening of lifespan of a battery due
to battery deterioration, and the like.
[0003] For this reason, the effective heat dissipation in an
electronic device is becoming one of very important tasks.
[0004] As a heat dissipation means used in an electronic device,
graphite having a superior thermal conductivity draws attention.
Among them, an artificial graphite sheet which can be easily
processed into a sheet form and has a thermal conductivity of about
2 to 7 times higher than the thermal conductivity of copper or
aluminum is in the spotlight.
[0005] Such an artificial graphite sheet can be obtained through a
carbonization process and a graphitization process of a polymer.
Among the polymers, a heat-resistant polymer capable of
withstanding a temperature of about 400 degrees C. or higher can be
used as a graphite precursor. A representative example of such a
heat-resistant polymer is polyimide (PI).
[0006] Polyimide, which is based on a rigid aromatic main chain and
an imide ring having an excellent chemical stability, is a polymer
material having the highest level of heat resistance, chemical
resistance, electrical insulation and weather resistance among
organic materials. Polyimide is known as an optimal graphite
precursor because polyimide makes it possible to achieve excellent
yield, crystallinity and thermal conductivity in the preparation of
an artificial graphite sheet.
[0007] In general, the physical properties of an artificial
graphite sheet are known to be greatly affected by the physical
properties of polyimide as a graphite precursor. Thus, the
modification of polyimide has been actively studied in order to
improve the physical properties of the artificial graphite sheet.
In particular, extensive research is underway to improve the
thermal conductivity of the artificial graphite sheet.
[0008] Nevertheless, there is no clear result in the development of
an artificial graphite sheet having a very high thermal
conductivity capable of remarkably improving the performance of an
electronic device due to heat dissipation and a polyimide capable
of realizing the artificial graphite sheet.
[0009] Therefore, it is necessary to develop an artificial graphite
sheet having a desired thermal conductivity and a polyimide capable
of realizing the artificial graphite sheet.
SUMMARY
[0010] It is an object of the present disclosure to provide a novel
polyimide film and a graphite sheet prepared by the polyimide
film.
[0011] According to one aspect of the present disclosure, the
diffraction peaks of (010) plane and (102) plane according to the
XRD analysis among the crystal planes of a polyimide film are
disclosed as essential factors required for realizing a graphite
sheet having a high thermal conductivity.
[0012] In particular, in the diffraction peaks of (010) plane and
(102) plane, the polyimide film may include the relationship
between FWHMs (full width at half maximum) thereof as an important
factor for realizing a graphite sheet having an excellent thermal
conductivity. Specifically, the polyimide film may include, as the
important factor, a value of the following Formula 1 which is
established to calculate the relationship between FWHMs as
quantitative value. When the calculated value satisfies a specific
numerical range in the present disclosure, the polyimide film may
be desirable for the realization of a graphite sheet having a
significantly excellent thermal conductivity.
[0013] According to another aspect of the present disclosure, a
method for preparing a graphite sheet may include a value of
Formula 1 as an important factor. A graphite sheet having a desired
high thermal conductivity can be prepared using a polyimide film in
which a value of Formula 1 falls with the above numerical
range.
[0014] Accordingly, the present disclosure has a practical purpose
in providing a specific embodiment thereof.
[0015] The present disclosure focuses on a polyimide film in which
a value of Formula 1 with respect to the relationship between FWHMs
is less than 0.4. Such a polyimide film and a method for preparing
a graphite sheet using the polyimide film may make it possible to
realize a graphite sheet having an extremely high thermal
conductivity of 1400 W/mK or more.
[0016] Accordingly, the present disclosure provides a polyimide
film which is prepared from a precursor composition including a
polyamic acid and an organic solvent and in which a value of the
following Formula 1 is less than 0.4, and a graphite sheet derived
from the polyimide film.
(first FWHM-second FWHM)/(first FWHM+second FWHM) Formula 1
[0017] The present disclosure also provides a method for preparing
a graphite sheet. Specifically, the method may include: preparing a
polyimide film having a value of Formula 1 is less than 0.4;
carbonizing the polyimide film; and obtaining a graphite sheet by
graphitizing the carbonized polyimide film.
[0018] Hereinafter, embodiments of the present disclosure will be
described in more detail in the order of a "polyimide film", a
"method for preparing a graphite sheet " and a "graphite sheet"
according to the present invention.
[0019] Terms and words used in the present specification and the
claims should not be construed as being limited to ordinary or
lexical meanings, but should be construed as meanings and concepts
consistent with the technical idea of the present disclosure based
on the principle that an inventor may appropriately define the
concepts of terms to describe its own invention in the best
way.
[0020] Therefore, the configurations of the embodiments disclosed
herein are nothing more than the most preferred embodiments of the
present disclosure and are not intended to represent all of the
technical ideas of the present disclosure. It is to be understood
that various equivalents and modifications of these configuration
may exist.
[0021] As used herein, singular forms include plural forms unless
the context clearly dictates otherwise. In this specification, the
terms "comprising", "including", "having" and the like are intended
to indicate the presence of features, numbers, steps, elements or
the combinations thereof, and are not intended to preclude the
presence or the possibility of addition of one or more other
features, numbers, steps, elements or the combinations thereof.
[0022] As used herein, the term "dianhydride" is intended to
encompass its precursors or derivatives, which may not technically
be dianhydride but may react with diamine to form a polyamic acid.
The polyamic acid may be converted into polyimide again.
[0023] As used herein, the term "diamine" is intended to encompass
its precursors or derivatives, which may not technically be diamine
but may react with dianhydride to form a polyamic acid. The
polyamic acid may be converted into polyimide again.
[0024] In this specification, when an amount, a concentration, or
other value or parameter is given as an enumeration of a range, a
preferred range, or a preferred upper limit value and a preferred
lower limit value, it is understood that the range specifically
discloses all ranges defined by a pair of an arbitrary upper range
limit value or a preferred value and an arbitrary lower range limit
value or a preferred value regardless of whether the range is
separately disclosed. When a range of numerical values is mentioned
in this specification, unless otherwise stated, the range is
intended to encompass the endpoints thereof and all the integers
and fractions falling within the range. The scope of the present
disclosure is intended not to be limited to the specific values
mentioned when defining the scope.
Polyimide Film
[0025] The polyimide film according the present disclosure may be
prepared from a precursor composition including a polyamic acid and
an organic solvent. The details of the polyimide film will be
described in detail through the following non-limiting
embodiments.
<Features of Polyimide Film>
[0026] In one specific embodiment, the polyimide film has a value
from the following Formula 1 which may fall within a numerical
value range of less than 0.4, specifically 0.01 or more and 0.37 or
less, more specifically 0.02 or more and 0.16 or less, particularly
specifically 0.03 or more and 0.09 or less. In the case of the
polyimide film of the present disclosure, the graphite sheet
prepared using the polyimide film may exhibit a high thermal
conductivity.
(first FWHM-second FWHM)/(first FWHM+second FWHM) Formula 1
[0027] In this regard, when a diffraction peak of (010) plane
according to XRD analysis of the polyimide film is in the form of a
single peak, the first FWHM may indicate a FWHM of the single peak;
and when the diffraction peak of (010) plane is in the form of a
plurality of single peaks or in the form of a plurality of single
peaks overlapped with each other, the first FWHM may indicate an
average of individual FWHMs of the single peaks.
[0028] When a diffraction peak of (102) plane according to XRD
analysis of the polyimide film is in the form of a single peak, the
second FWHM may indicate a FWHM of the single peak; and when the
diffraction peak of (102) plane is in the form of a plurality of
single peaks or in the form of a plurality of single peaks
overlapped with each other, the second FWHM may indicate an average
of individual FWHMs of the single peaks.
[0029] It is believed in the present disclosure that among the
crystal planes of the polyimide film, the diffraction peak of (010)
plane and the diffraction peak of (102) plane are closely related
to the overall orientation of the film. In the case where the
orientation of the polyimide film is excellent, a high thermal
conductivity may be exhibited in the graphite sheet derived from
the polyimide film.
[0030] Generally, the larger the width of the diffraction peak, the
lower the degree of crystallinity (or the orientation). The smaller
the width of the diffraction peak, the higher the degree of
crystallinity (or the orientation). The quantitative analysis of
the diffraction peak width may be determined based on a FWHM of a
peak.
[0031] However, even if each of the first FWHM and the second FWHM
for the diffraction peak of (010) plane and the diffraction peak of
(102) plane is relatively small, when the difference between the
first FWHM and the second FWHM is too large or too small, the first
FWHM and the second FWHM may deviate from the numerical value range
of the present disclosure related to the value of Formula 1. In the
case of preparing a graphite sheet using a conventional polyimide
film corresponding to this case, a high thermal conductivity may
not be exhibited.
[0032] In contrast, even if each of the first FWHM and the second
FWHM for the diffraction peak of (010) plane and the diffraction
peak of (102) plane is relatively large, the value of Formula 1 may
fall within the numerical value range described above. In this
case, the graphite sheet prepared from the polyimide film according
to the present disclosure may have a relatively high thermal
conductivity.
[0033] In other words, in order to achieve a high thermal
conductivity in the graphite sheet, it is important that the value
of Formula 1 satisfies the numerical value range of the present
disclosure while the first FWHM and the second FWHM have
appropriate values. This will be clearly demonstrated below. The
polyimide film of the present disclosure has the value of Formula 1
which satisfies the numerical value range of the present
disclosure. Thus, when a graphite sheet is prepared from the
polyimide film, it is possible to realize a graphite sheet having a
high thermal conductivity of 1400 W/mK or more.
[0034] Meanwhile, it is considered that the orientation of the
polyimide film is related to various conditions such as the type of
a monomer, the solid content of a precursor composition, the
viscosity, the casting temperature of the polyimide film, the
degree of elongation of a film intermediate, and the like. Even if
some of these conditions are satisfied, the orientation is not
particularly improved, or a high thermal conductivity is not
exhibited when preparing a graphite sheet. Surprisingly, however,
even if there are some differences in the conditions relating to
the orientation, the graphite sheet prepared from the polyimide
film having the value of Formula 1 which satisfies the numerical
value range of the present disclosure may exhibit a high thermal
conductivity. As a result, it can be concluded that the diffraction
peak of (010) plane and the diffraction peak of (102) plane are
closely related to the orientation.
[0035] It is also possible to quantitatively calculate the
relationship between the first FWHM and the second FWHM for the
diffraction peak of (010) plane and the diffraction peak of (102)
plane by Formula 1. It is possible to qualitatively predict the
thermal conductivity of the graphite sheet on the basis of whether
the calculated value falls within the numerical value range of the
present disclosure or not.
[0036] It is presumed that the above advantage is due to the
relatively advantageous effect of carbon rearrangement during
carbonization and graphitization of the polyimide film when the
value of Formula 1 falls within the specific numerical value range
of the present disclosure.
[0037] Further, if it is assumed that the graphitization proceeds
substantially simultaneously in the surface layer and the inside of
the polyimide film, the polyimide film according to the present
disclosure satisfying the above-mentioned specific numerical value
range has orientation characteristics suitable for conversion into
a graphite sheet. Thus, it is expected that the polyimide film has
suitable orientation which assists in forming a graphite structure
having a high level of thermal conductivity both in the surface
layer and the inside of the polyimide film and in suppressing a
phenomenon that a gas generated from the inside of the polyimide
film destroys the graphite structure formed in the surface
layer.
[0038] In one embodiment, the first FWHM may be 35 degrees or more
and 80 degrees or less. The second FWHM may be 43% or more and 92%
or less, more specifically a value of 49% or more and 92% or less,
of the first FWHM.
[0039] <Precursor Composition>
[0040] In one specific embodiment, the polyamic acid may be
prepared by the polymerization reaction of at least one kind of
diamine monomers and at least one kind of dianhydride monomers
(acid dianhydride).
[0041] The dianhydride monomer which can be used for the
preparation of the polyamic acid may include pyromellitic
dianhydride, 2,3,6,7-naphthalene tetracarboxylic dianhydride,
3,3',4,4'-biphenyl tetracarboxylic dianhydride, 1,2,5,6-naphthalene
tetracarboxylic dianhydride, 2,2',3,3'-biphenyl tetracarboxylic
dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride,
2,2-bis(3,4-dicarboxyphenyl) propane dianhydride, 3,4,9,10-perylene
tetracarboxylic dianhydride, bis(3,4-(dicarboxyphenyl) propane
dianhydride, 1,1-bis(2,3-dicarboxyphenyl) ethane dianhydride,
1,1-bis (2,3-dicarboxyphenyl) ethane dianhydride,
1,1-bis(3,4-dicarboxyphenyl) ethane dianhydride,
bis(2,3-dicarboxyphenyl) methane dianhydride,
bis(3,4-dicarboxyphenyl) ethane dianhydride, oxydiphthalic
anhydride, bis(3,4-dicarboxyphenyl) sulfone dianhydride,
p-phenylene bis (trimellitic monoester acid anhydride), ethylene
bis(trimellitic monoester acid anhydride, bisphenol A bis
(trimellitic monoester acid anhydride), and analogues thereof.
These substances may be used alone or as a mixture of the
substances mixed in an arbitrary ratio.
[0042] The diamine which can be used for the preparation of the
polyamic acid may include 4,4'-diaminodiphenylpropane,
4,4'-diaminodiphenylmethane, benzidine, 3,3'-dichlorobenzidine,
4,4'-diaminodiphenylsulfide, 3,3'-diaminodiphenylsulfone,
4,4'-diaminodiphenylsulfone,
4,4'-diaminodiphenylether(4,4'-oxydianiline),
3,3'-diaminodiphenylether(3,3'-oxydianiline),
3,4'-diaminodiphenylether(3,4'-oxydianiline),
1,5-diaminonaphthalene, 4,4'-diaminodiphenyldiethylsilane,
4,4'-diaminodiphenylsilane, 4,4'-diaminodiphenylethylphosphine
oxide, 4,4'-diaminodiphenyl N-methylamine, 4,4'-diaminodiphenyl
N-phenylamine, 1,4-diaminobenzene(p-phenylenediamine),
1,3-diaminobenzene, 1,2-diaminobenzene, and analogues thereof.
These substances may be used alone or as a mixture of the
substances mixed in an arbitrary ratio.
[0043] The organic solvent is not particularly limited, and any
solvent capable of dissolving the polyamic acid may be used.
However, an amide-based solvent is preferable. Specifically, the
solvent may be an organic polar solvent and may be an aprotic polar
solvent. For example, the solvent may be, but is not limited to,
one or more selected from the group consisting of
N,N'-dimethylformamide (DMF), N,N'-dimethylacetamide,
N-methylpyrrolidone (NMP), gammabutyrolactone (GBL) and diglyme. If
necessary, these substances may be used alone or in combination. In
one example, N,N'-dimethylformamide and N,N'-dimethylacetamide may
be particularly preferably used as the solvent.
[0044] Optionally, the precursor composition may further include
fillers such as calcium carbonate, dicalcium phosphate, barium
sulfate, and the like.
<Preparation of Polyimide Film>
[0045] In one specific embodiment, the precursor composition for
the preparation of the polyimide film may satisfy the following
conditions (a) to (c):
[0046] (a) a viscosity of the precursor composition at 23 degrees
C. is from 55,000 cP to 900,000 cP;
[0047] (b) a heat treatment temperature for heating and drying the
precursor composition after casting the precursor composition is
from 150 degrees C. to 200 degrees C.; and
[0048] (c) a film intermediate derived from the precursor
composition is stretched in at least one of a machine direction
(MD) and a transverse direction (TD).
[0049] Depending on the above conditions, the physical properties
of the polyimide film may be changed, and each of the first half
width and the second half width may be changed. The significance of
the above conditions (a) to (c) will be described below in more
detail.
i) Condition (a)
[0050] The weight average molecular weight of the polyamic acid
contained in the precursor composition may be 240,000 or more,
specifically 260,000 or more, and more specifically, 280,000 or
more. The use of the polyimide film prepared from the precursor
composition may be advantageous for the preparation of a graphite
sheet having an excellent thermal conductivity because carbon
chains longer and larger than those when weight average molecular
weight is less than 240,000 are polymerized.
[0051] The weight average molecular weight in the above range may
be controlled by adjusting the viscosity of the precursor
composition. Specifically, the weight average molecular weight
increases in proportion to the viscosity. However, the weight
average molecular weight is not linearly proportional to the
viscosity but is proportional to the viscosity in a logarithmic
form.
[0052] That is, even if the viscosity is increased to obtain a
polyamic acid having a higher weight average molecular weight, the
range in which the weight average molecular weight can increase is
limited. If the viscosity is excessively high, when varnish is
discharged through a die in a process of casting for preparing a
polyimide film, a problem of workability may be caused due to an
increase in the pressure inside the die or the like.
[0053] In the present disclosure, the polyamic acid may be
contained in an amount of 15 to 20% by weight based on the total
weight of the precursor composition. The weight average molecular
weight in the above range may be achieved by controlling the
viscosity in this range.
[0054] In this case, the content of the polyamic acid corresponds
to the total amount of the dianhydride monomers and the diamine
monomers used in the reaction. In other words, the content of the
polyamic acid may be referred to as "the solid content of the
polyamic acid" present in the precursor composition.
[0055] A more preferred range of the viscosity may be 90,000 cP to
300,000 cP, more preferably 100,000 cP to 250,000 cP.
ii) Condition (b)
[0056] After casting the precursor composition, as the precursor
composition is subjected to a heat treatment at a predetermined
temperature conforming to the condition (b), the polyamic acid may
be partially cyclized and dehydrated to form imide rings, whereby a
film intermediate may be formed.
[0057] As a method for imidizing the precursor composition, a
conventionally known method may be used. Examples of the
conventionally known method may include a thermal imidizing method,
a chemical imidizing method, or a composite imidizing method using
the thermal imidizing method and the chemical imidizing method in
combination.
[0058] The thermal imidizing method is a method in which an
imidizing reaction proceeds only by heating without using a
dehydrating/ring-closing agent or the like. The thermal imidizing
method is a method in which a polyimide film having a imidized
polyamic acid is obtained by gradually increasing a temperature in
a predetermined temperature range after casting a polyamic acid on
a support body.
[0059] The chemical imidizing method is a method of accelerating
imidization by causing a chemical conversion agent and/or an
imidizing catalyst to act on a precursor composition.
[0060] The composite imidizing method is a method in which a
polyimide film is obtained by adding a dehydrating agent and an
imidizing catalyst to a precursor composition, casting the
precursor composition on a support body, heating the precursor
composition at a predetermined temperature to activate the
dehydrating agent and the imidizing catalyst, partially curing and
drying the precursor composition, and then heating the precursor
composition again.
[0061] In the present disclosure, the polyimide film may be
prepared by appropriately selecting one of the imidizing methods
described above. The following description will be made under the
assumption that the most commonly used chemical imidizing method is
used.
[0062] In order to prepare the polyimide film, a chemical
conversion agent and/or an imidizing catalyst may be first mixed
into a precursor composition at a low temperature.
[0063] The chemical conversion agent and the imidizing catalyst are
not particularly limited. For example, beta-picoline, acetic
anhydride, dibasic calcium phosphate and the like may be used.
[0064] Meanwhile, the precursor composition is cast in a film form
on a support body such as a glass plate, an aluminum foil, an
endless stainless steel belt, a stainless steel drum or the like.
Thereafter, the precursor composition on the support body is heated
in a temperature range of 150 degrees C. to 200 degrees C. By doing
so, the chemical conversion agent and the imidizing catalyst are
activated. Partial curing and/or drying occur, thereby forming an
intermediate in a pre-filming step. Thereafter, the film is peeled
from the support body to obtain a film intermediate.
[0065] Since the film intermediate thus obtained is imidized at a
relatively low temperature, it may have a self-supporting property
and may be in the form of a gel favorable for stretching.
iii) Condition (c)
[0066] The film intermediate may be stretched to adjust the
thickness and size of the polyimide film prepared therefrom and to
improve the orientation. At this time, the stretching may be
performed in at least one of a machine direction (MD) and a
transverse direction (TD) with respect to the machine
direction.
[0067] When the film intermediate is stretched in the machine
direction, the molecular orientation may follow the machine
direction so that the orientation of the polyimide film may be
improved. However, when the stretching of the film intermediate is
carried out for the purpose of orientation in any one direction,
for example, orientation in the machine direction, shrinkage in the
transverse direction, which is the other direction, may be
accompanied. Thus, the stretching direction and stretching ratio
should be carefully selected.
[0068] Therefore, in the present disclosure, the stretching ratio
in at least one of the machine direction and the transverse
direction may be +3% or more and +25% or less. Specifically, the
stretching ratio in both the machine direction and the transverse
direction may be +3% or more and +25% or less.
[0069] In addition, the film intermediate may be stretched so that
the polyimide film has a thickness of from 20 micrometers to 125
micrometers.
[0070] When the thickness of the polyimide film is less than the
above-mentioned range, it is not preferable in that an excessively
thin graphite sheet may be obtained. When the thickness of the
polyimide film exceeds the above-mentioned range, the thermal
energy for preparing a graphite sheet may be excessively consumed,
and the graphitized form may be different in the surface of the
polyimide film and the inside of the polyimide film, which is
undesirable.
[0071] In the meantime, the film intermediate partially imidized
together with the stretching may be further heat-treated to
completely imidize the film intermediate. The heat treatment for
the complete imidization may be performed at 250 degrees C. to 850
degrees C. for 5 to 25 minutes.
Method for Preparing a Graphite Sheet
[0072] The method for preparing a graphite sheet according to the
present disclosure may include: preparing a polyimide film having a
value of the following Formula 1 which is less than 0.4;
carbonizing the polyimide film; and obtaining a graphite sheet by
graphitizing the carbonized polyimide film, wherein the graphite
sheet has a thermal conductivity of 1400 W/mK or more.
(first FWHM-second FWHM)/(first FWHM+second FWHM) Formula 1
[0073] In this regard, the definitions of the first FWHM and the
second FWHM are the same as those described in the preceding
embodiment of the polyimide film.
[0074] The method for preparing a graphite sheet of the present
disclosure has been made by noting the fact that among the crystal
planes of the polyimide film as a raw material for preparation, in
particular, the diffraction peak of (010) plane and the diffraction
peak of (102) plane are related to the orientation of the film.
[0075] When the polyimide film has a specific value of Formula 1
for the relationship between FWHMs of the diffraction peaks, the
graphite sheet prepared therefrom may exhibit a considerably high
thermal conductivity of 1400 W/mK or more, specifically 1500 W/mK
or more.
[0076] Presumably, this is because the carbon rearrangement is
relatively advantageous in the carbonization process of the
polyimide film when the value of Formula 1 falls within the
specific numerical value range of the present disclosure,
specifically less than 0.4, as described above. Assuming that the
graphitization proceeds almost simultaneously in the surface layer
and the inside of the polyimide film, the polyimide film according
to the present disclosure satisfying the above numerical value
range has the orientation characteristic suitable for conversion
into a graphite sheet. Thus, it is expected that the polyimide film
has suitable orientation which assists in forming a graphite
structure having a high level of thermal conductivity both in the
surface layer and the inside of the polyimide film and in
suppressing a phenomenon that a gas generated from the inside of
the polyimide film destroys the graphite structure formed in the
surface layer.
[0077] In one specific embodiment, the value of Formula 1 may fall
within a numerical value range 0.01 or more and 0.37 or less, more
specifically 0.02 or more and 0.16 or less, particularly
specifically 0.03 or more and 0.09 or less.
[0078] In one embodiment, the first FWHM may be 35 degrees or more
and 80 degrees or less. The second FWHM may be a value of 43% or
more and 92% or less, more specifically a value of 49% or more and
92% or less, with respect to the first FWHM.
[0079] The obtaining the graphite sheet may include carbonizing the
polyimide film and graphitizing the carbonized film.
[0080] The carbonizing the polyimide film may be performed using a
hot press and/or an electric furnace under a reduced pressure or in
a nitrogen gas. In the present disclosure, the carbonizing the
polyimide film may be carried out at a temperature of about 1,000
degrees C. to 1,500 degrees C. for about 1 hour to 5 hours.
[0081] In some cases, a pressure may be applied in a vertical
direction using a hot press for carbon orientation in a desired
form. In this case, a pressure of 5 kg/cm.sup.2 or more,
specifically 15 kg/cm.sup.2 or more, more specifically 25
kg/cm.sup.2 or more may be applied during the carbonizing process.
However, this is nothing more than an example for helping to carry
out the present disclosure, and the above pressure condition does
not limit the scope of the present disclosure.
[0082] Subsequently, the graphitizing the carbonized polyimide film
may be performed.
[0083] The graphitizing the carbonized polyimide film may also be
performed using a hot press and/or an electric furnace. The
graphitizing the carbonized polyimide film may also be performed in
an inert gas. Preferred examples of the inert gas may include a
mixed gas containing argon and a small amount of helium.
[0084] The heat treatment temperature in the graphitizing the
carbonized polyimide film is required to be at least 2,500 degrees
C. or more, and may preferably be 3,000 degrees C. or less in
consideration of economy.
[0085] In some cases, a pressure of 100 kg/cm.sup.2 or more,
specifically 200 kg/cm.sup.2 or more, more specifically 300
kg/cm.sup.2 or more may be applied in the graphitizing the
carbonized polyimide film. However, this is nothing more than an
example for helping to carry out the present disclosure, and the
above pressure condition does not limit the scope of the present
disclosure.
[0086] In the method for preparing a graphite sheet according to
the present disclosure, the preparation of a polyimide film may
satisfy the following process conditions (a) to (c):
[0087] (a) the viscosity of the precursor composition containing a
polyamic acid and an organic solvent is 55,000 cP to 900,000;
[0088] (b) a heat treatment for heating and drying the precursor
composition after casting the precursor composition is conducted at
a temperature of from 150 degrees C. to 200 degrees C.; and
[0089] (c) a film intermediate prepared by the heat treatment is
stretched at least once to prepare the polyimide film.
[0090] The significance of the process conditions (a) to (c) may be
the same as the conditions (a) to (c) of the polyimide film
according to the present disclosure described above.
[0091] Specifically, the preparation of a polyimide film may
comprises:
[0092] preparing the precursor composition, comprising the process
condition (a);
[0093] forming the film intermediate by heat-treating the precursor
composition, comprising the process condition (b); and
[0094] stretching the film intermediate in at least one of a
machine direction and a transverse direction, comprising the
process condition (c).
[0095] Examples of the method for preparing and polymerizing the
precursor composition are as follows.
[0096] (1) A method in which the whole amount of diamine monomers
is put into a solvent, and then dianhydride monomers are added in a
mole substantially equal to the mole of the diamine monomers,
thereby performing polymerization.
[0097] (2) A method in which the whole amount of dianhydride
monomers is put into a solvent, and then diamine monomers are added
in a mole substantially equal to the mole of the dianhydride
monomers, thereby performing polymerization.
[0098] (3) A method in which some of diamine monomers are put into
a solvent, some of dianhydride monomers are mixed in a ratio of
about 95 to 105 mol % with respect to the reacted diamine monomers,
the remaining diamine monomers are added, and then the remaining
dianhydride monomers are added so that the total mole of the
diamine monomers and the total mole of the dianhydride monomers are
substantially equal to each other, thereby performing
polymerization.
[0099] (4) A method in which some of dianhydride monomers are put
into a solvent, some of diamine monomers are mixed in a ratio of
about 95 to 105 mol % with respect to the reacted dianhydride
monomers, the remaining dianhydride monomers are added, and then
the remaining diamine monomers are added so that the total mole of
the diamine monomers and the total mole of the dianhydride monomers
are substantially equal to each other, thereby performing
polymerization.
[0100] (5) A method in which a first precursor composition is
formed by reacting some of diamine monomers and some of dianhydride
monomers in a solvent so that the amount of one of the diamine
monomers and the dianhydride monomers becomes excessive, a second
precursor composition is formed by reacting some of diamine
monomers and some of dianhydride monomers in another solvent so
that the amount of one of the diamine monomers and the dianhydride
monomers becomes excessive, and then the first precursor
composition and the second precursor composition are mixed to
perform polymerization, wherein the amount of the dianhydride
monomers is made excessive in the second precursor composition when
the amount of the diamine monomers is excessive during the
formation of the first precursor composition, the amount of the
diamine monomers is made excessive in the second precursor
composition when the amount of the dianhydride monomers is
excessive in the first precursor composition, and the first
precursor composition and the second precursor composition are
mixed to perform polymerization so that the total mole of the
diamine monomers and the total mole of the dianhydride monomers are
equal to each other.
[0101] However, the precursor composition polymerization method is
not limited to the above, and any known methods may be used.
[0102] During the polymerization, the weight average molecular
weight of the polyamic acid contained in the precursor composition
may be 240,000 or more, specifically 260,000 or more, and more
specifically, 280,000 or more. The use of the polyimide film
prepared from the precursor composition may be advantageous for the
preparation of a graphite sheet having an excellent thermal
conductivity because carbon chains longer and larger than those
when weight average molecular weight is less than 240,000 are
polymerized.
[0103] The weight average molecular weight in the above range may
be controlled by adjusting the viscosity of the precursor
composition. Specifically, the weight average molecular weight
increases in proportion to the viscosity. However, the weight
average molecular weight is not linearly proportional to the
viscosity but is proportional to the viscosity in a logarithmic
form.
[0104] That is, even if the viscosity is increased to obtain a
polyamic acid having a higher weight average molecular weight, the
range in which the weight average molecular weight can increase is
limited. If the viscosity is excessively high, when varnish is
discharged through a die in a process of casting for preparing a
polyimide film, a problem of workability may be caused due to an
increase in the pressure inside the die or the like.
[0105] Thus, the precursor composition may be prepared by mixing
the polyamic acid in an amount of 15 to 20% by weight and the
organic solvent in an amount of 80 to 85% by weight based on the
total weight of the precursor composition. The weight average
molecular weight in the above range may be achieved by controlling
the viscosity in this range.
[0106] In this case, the content of the polyamic acid corresponds
to the total amount of the dianhydride monomers and the diamine
monomers used in the reaction. The content of the polyamic acid may
be referred to as "the solid content of the polyamic acid" present
in the precursor composition.
[0107] A more preferred range of the viscosity may be 90,000 cP to
300,000 cP, more preferably 100,000 cP to 250,000 cP.
[0108] The dianhydride monomers, the diamine monomers and the
organic solvent may be optionally used in the examples described
above.
[0109] In the process of preparing the film intermediate by
heat-treating the precursor composition, after casting the
precursor composition on a support body, as the precursor
composition is subjected to a heat treatment at a predetermined
temperature conforming to the condition (b), the polyamic acid may
be partially cyclized and dehydrated to form imide rings, whereby a
film intermediate may be formed.
[0110] As the method for imidizing the precursor composition, a
conventionally known method may be used. Examples of the
conventionally known method may include a thermal imidizing method,
a chemical imidizing method, or a composite imidizing method using
the thermal imidizing method and the chemical imidizing method in
combination. These methods are the same as described above and,
therefore, will not be described again.
[0111] The stretching the film intermediate is performed to adjust
the thickness and size of the polyimide film and to improve the
orientation by stretching the film intermediate. The film
intermediate may be stretched at a temperature of 20 to 40 degrees
C. so as to have a thickness of 20 to 125 micrometers. In this
case, the stretching ratio in at least one of the machine direction
and the transverse direction may be +3% or more and +25% or less.
Specifically, the stretching ratio in both the machine direction
and the transverse direction may be between +3% or more and +25% or
less.
Graphite Sheet
[0112] The present disclosure provides a graphite sheet prepared
from the polyimide film, or a graphite sheet prepared by the above
preparation method.
[0113] The graphite sheet may have a thermal conductivity of 1400
W/mK or more, specifically 1500 W/mK or more. Accordingly, when the
graphite sheet is used as a heat dissipating means of an electronic
device, the graphite sheet may accelerate heat dissipation and may
significantly contribute to the improvement of performance
improvement of an electronic device.
[0114] The graphite sheet may have a thickness of 15 .mu.m to 60
.mu.m.
[0115] If the thickness of the graphite sheet is out of the above
range and is too thin or too thick, a desired range of thermal
conductivity may not be exhibited. Furthermore, the handling and
molding of the graphite sheet may not be easy in the process of
applying the graphite sheet to a desired electronic device or the
like.
[0116] The present disclosure also provides an electronic device
including the high performance graphite sheet described above. The
specific kind, configuration and structure of the electronic device
are well known in the art. Therefore, detailed description thereof
will be omitted.
[0117] The present disclosure has been specifically described above
to indicate that the relationship between FWHMs of diffraction
peaks according to the XRD analysis of a polyimide film and the
thermal conductivity of a graphite sheet prepared from the
polyimide film are significantly related to each other.
[0118] In summary, a value of Formula 1 which can quantitatively
establish the relationship between FWHMs of diffraction peaks on a
(010) plane and (102) plane of a polyimide film may be used to
improve the thermal conductivity of a graphite sheet. Thus, in the
polyimide film of the present disclosure, the value of Formula 1
may be less than 0.4, specifically 0.01 to 0.37, more specifically
0.02 to 0.16, even more specifically 0.03 to 0.09.
[0119] Accordingly, the polyimide film and the method for preparing
the graphite sheet using the same may make it possible to realize a
graphite sheet having a high thermal conductivity.
[0120] In addition, the graphite sheet prepared according to the
method for preparing the graphite sheet of the present disclosure
has a thermal conductivity of 1400 W/mK or more, specifically 1500
W/mK or more. Thus, the graphite sheet may remarkably improve the
performance of an electronic device based on the extremely high
thermal conductivity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0121] FIG. 1 is a graph showing the XRD result for a (010) plane
of a polyimide film according to example 1.
[0122] FIG. 2 is a graph showing the XRD result for (102) plane of
a polyimide film according to example 1.
DETAILED DESCRIPTION
[0123] Hereinafter, the action and effect of the present disclosure
will be described in more detail through specific examples of the
present disclosure. It is to be understood, however, that such
examples are merely illustrative of the present disclosure and are
not intended to limit the scope of the present disclosure.
Preparation Example of Precursor Composition
PREPARATION EXAMPLE 1
[0124] 400 g of dimethylformamide was put into a 0.5 L reactor and
the temperature was set to 20 degrees C. Then, 33.79 g of
4,4-diaminodiphenylether was added and dissolved. Thereafter, 35.33
g of pyromellitic dianhydride was added and dissolved. When
dissolution was completed, pyromellitic dianhydride was gradually
added to the solution, and the viscosity was measured to obtain a
varnish of about 100,000 cP. At this time, the polyamic acid solid
content is 15% based on the total amount of the precursor
composition.
PREPARATION EXAMPLE 2
[0125] A precursor composition was obtained in the same manner as
in preparation example 1 except that a varnish of about 250,000 cP
was obtained. At this time, the polyamic acid solid content is 15%
based on the total amount of the precursor composition.
PREPARATION EXAMPLE 3
[0126] A precursor composition was obtained in the same manner as
in preparation example 1 except that a varnish of about 400,000 cP
was obtained. At this time, the polyamic acid solid content is 15%
based on the total amount of the precursor composition.
PREPARATION EXAMPLE 4
[0127] A precursor composition was obtained in the same manner as
in preparation example 1 except that a varnish of about 500,000 cP
was obtained. At this time, the polyamic acid solid content is 15%
based on the total amount of the precursor composition.
PREPARATION EXAMPLE 5
[0128] A precursor composition was obtained in the same manner as
in preparation example 1 except that a varnish of about 500,000 cP
was obtained and polymerization was performed so that the polyamic
acid solid content is 20% based on the total amount of the
precursor composition.
PREPARATION EXAMPLE 6
[0129] A precursor composition was obtained in the same manner as
in preparation example 1 except that a varnish of about 900,000 cP
was obtained and polymerization was performed so that the polyamic
acid solid content is 15% based on the total amount of the
precursor composition.
PREPARATION EXAMPLE 7
[0130] A precursor composition was obtained in the same manner as
in preparation example 1 except that a varnish of about 30,000 cP
was obtained and polymerization was performed so that the polyamic
acid solid content is 15% based on the total amount of the
precursor composition.
PREPARATION EXAMPLE 8
[0131] A precursor composition was obtained in the same manner as
in preparation example 1 except that a varnish of about 30,000 cP
was obtained and polymerization was performed so that the polyamic
acid solid content is 10% based on the total amount of the
precursor composition.
PREPARATION EXAMPLE 9
[0132] A precursor composition was obtained in the same manner as
in preparation example 1 except that a varnish of about 30,000 cP
was obtained and polymerization was performed so that the polyamic
acid solid content is 25% based on the total amount of the
precursor composition.
EXAMPLE
Example 1
[0133] 4.5 g of beta-picoline (boiling point: 144 degrees C.) as an
imide curing catalyst, 17.0 g of acetic anhydride as a dehydrating
agent and 23.5 g of dimethylformamide as a polar organic solvent
were mixed and stirred in 100 g of the precursor composition
obtained in preparation example 1. After mixing 45 g of imide
conversion liquid with the solution, thus obtained, the solution
was cast on a stainless steel plate and was dried in a 150 degrees
C. oven for 4 minutes with a hot air. Thereafter, the film thus
obtained was stretched by +3% in the machine direction and the
transverse direction, respectively, at a temperature of 20 degrees
C. to obtain a polyimide film.
Example 2
[0134] A polyimide film was obtained in the same manner as in
example 1 except that the precursor composition obtained in
preparation example 2 was used.
Example 3
[0135] A polyimide film was obtained in the same manner as in
example 1 except that the precursor composition obtained in
preparation example 3 was used.
Example 4
[0136] A polyimide film was obtained in the same manner as in
example 1 except that the precursor composition obtained in
preparation example 4 was used.
Example 5
[0137] A polyimide film was obtained in the same manner as in
example 1 except that the precursor composition obtained in
preparation example 5 was used.
Example 6
[0138] A polyimide film was obtained in the same manner as in
example 1 except that the precursor composition obtained in
preparation example 2 was used and the film was stretched by
.+-.25% in the machine direction and the transverse direction,
respectively.
Example 7
[0139] A polyimide film was obtained in the same manner as in
example 1 except that the precursor composition obtained in
preparation example 6 was used.
Comparative Example 1
[0140] A polyimide film was obtained in the same manner as in
example 1 except that the precursor composition obtained in
preparation example 7 was used.
Comparative Example 2
[0141] A polyimide film was obtained in the same manner as in
example 1 except that the precursor composition obtained in
preparation example 8 was used.
Comparative Example 3
[0142] A polyimide film was obtained in the same manner as in
example 1 except that the precursor composition obtained in
preparation example 9 was used.
Comparative Example 4
[0143] A polyimide film was obtained in the same manner as in
example 1 except that the film was stretched by +50% in the machine
direction and the transverse direction, respectively.
Experimental Example 1
Evaluation of Morphological Stability of Polyimide Film
[0144] In the polyimide films obtained in example 1 and comparative
example 4, the degree of shrinkage in the transverse direction
which occurs when stretching in the machine direction was
evaluated.
[0145] In the evaluation, the film width before stretching was
measured and the film width after stretching was measured to
calculate a shrinkage percentage. The shrinkage percentage in the
transverse direction (TD) as a result is shown in Table 1 below.
The breakage of the film was visually confirmed, and the result is
also shown in Table 1.
TABLE-US-00001 TABLE 1 TD shrinkage percentage (%) Breakage Example
1 1 X Comparative Example 4 18 .largecircle. (film broken)
[0146] Referring to Table 1, it can be seen that the polyimide film
according to example 1 has a small shrinkage percentage in the
transverse direction caused by the stretching in the machine
direction. As a result, it can be noted that there is no problem
such as breakage in the transverse direction stretching performed
after the machine direction stretching.
[0147] Accordingly, the polyimide film according to example 1 has
an advantage of improving the orientation by proper stretching and
dos not show any apparent breakage. Thus, the polyimide film can be
used for the preparation of a graphite sheet. As a result, it is
expected that a graphite sheet having excellent physical properties
can be prepared.
[0148] On the other hand, in the case of comparative example 4 in
which the stretching in the machine direction was excessively
performed, it can be noted that the transverse direction shrinkage
percentage increases at the time of machine direction stretching,
and the film is broken at the time of performing the transverse
direction stretching again.
[0149] This indicates that if the polyimide film is excessively
stretched beyond the stretching range of the present disclosure,
severe damage to the outer appearance may occur, making it
impossible to prepare a graphite sheet.
Experimental Example 2
Evaluation of Physical Properties of Polyimide Film
[0150] X-ray diffraction (XRD) analysis was performed on the
polyimide films obtained in examples 1 to 6 and comparative
examples 1 to 3 to measure the diffraction intensities with respect
to the azimuthal angles on the crystal planes (010) and (102).
Detailed conditions related to the XRD analysis are as follows.
[0151] Light source: flexural magnet synchrotron/6D UNIST-PAL beam
line (Pohang radiation accelerator)
[0152] Energy used: 18.986 keV (wavelength: 0.653 .ANG.)
[0153] Light source size: 100 (H).times.40 (V) um.sup.2
[0154] X-ray exposure time: 60 to 240 seconds
[0155] Detector: Rayonix MX225-HS (2880.times.2880 pixels, pixel
size: 78 um)
[0156] Based on this, a first FWHM of (010) plane and a second FWHM
of (102) plane were measured, and a value of the following Formula
1 was calculated using the first FWHM and the second FWHM. The
results are shown in Table 2.
(first FWHM-second FWHM)/(first FWHM+second FWHM) Formula 1
TABLE-US-00002 TABLE 2 First FWHM Second FWHM Calculated (degrees)
(degrees) value Example 1 78.9 36.6 0.37 Example 2 55.8 37.2 0.20
Example 3 59.0 43.9 0.15 Example 4 53.1 41.9 0.12 Example 5 57.9
41.1 0.17 Example 6 44.3 38.3 0.07 Example 7 65.1 59.6 0.04
Comparative example 1 85.4 35.1 0.42 Comparative example 2 84.5
33.1 0.44 Comparative example 3 88.5 34.0 0.44
[0157] As a result of the XRD analysis, it was confirmed that the
polyimide films according to examples 1 to 6 have the value of
Formula 1 which falls within the specific numerical value range
according to the present disclosure, i.e., less than 0.4, and the
polyimide films according to the comparative examples has the value
of Formula 1 which falls outside the specific numerical value range
according to the present disclosure.
[0158] Meanwhile, FIG. 1 shows an XRD analysis graph of (010) plane
of the polyimide film according to example 1. Referring to FIG. 1,
the difference between the maximum azimuth angle and the minimum
azimuth angle at (010) plane diffraction intensity of 50% can be
calculated as a FWHM. The FWHM of the diffraction peak may be the
first FWHM for (010) plane, the value of which is 78.9 degrees.
[0159] FIG. 2 shows an XRD analysis graph of (102) plane of the
polyimide film according to example 1.
[0160] It can be noted that the diffraction peak of (102) plane is
also has a form in which a plurality of peaks are superimposed. The
plurality of peaks may be separated into individual peaks by
calculation.
[0161] In each of the separated diffraction peaks, the difference
between the maximum azimuth angle and the minimum azimuth angle at
the diffraction intensity of 50% can be calculated as a FWHM. The
average of the FWHMs of the diffraction peaks may be the second
FWHM for (102) plane, the value of which is 36.6 degrees.
Experimental Example 3
Evaluation of Physical Properties of Graphite Sheet
[0162] The polyimide films obtained in examples 1 to 6 and
comparative examples 1 to 3 were heated to 1,200 degrees C. at a
rate of 1 degree C./min under the presence of a nitrogen gas using
an electric furnace capable of carbonization and were maintained
for about 2 hours (carbonization). Then, the polyimide films were
heated to 2,800 degrees C. at a rate of 20 degree C./min under the
presence of an argon gas using an electric furnace and were
maintained for about 8 hours. Then, the polyimide films were cooled
to obtain graphite sheets.
[0163] The thermal diffusivity of each of the graphite sheets was
measured by a laser flash method using a thermal diffusivity
measuring instrument (Model LFA 447, Netzsch Korea co., ltd.). The
thermal diffusivity thus measured was multiplied by the density
(weight/volume) and the specific heat (theoretical value: 0.85
kJ/kg.K) to calculate the thermal conductivity. The results are
shown in Table 3 below.
TABLE-US-00003 TABLE 3 Thermal diffusivity Thermal conductivity
(m.sup.2/s) (W/m K) Example 1 782.5 1410 Example 2 781.7 1422
Example 3 813.6 1473 Example 4 825.2 1487 Example 5 811.2 1441
Example 6 841.4 1531 Example 7 873.9 1560 Comparative example 1
706.4 1315 Comparative example 2 767.3 1350 Comparative example 3
702.6 1284
[0164] From the results shown in Table 3, it can be seen that the
graphite sheets prepared from the polyimide films of the examples
has significantly high thermal conductivity and thermal diffusivity
as compared with those of the comparative examples.
[0165] This clearly indicates that the polyimide films having the
value of Formula 1 which falls within the specific numerical value
range according to the present disclosure are capable of realizing
excellent thermal conductivity, and further that the polyimide
films having the value of Formula 1 which falls outside the
specific numerical value range according to the present disclosure
cannot realize graphite sheets having a desired thermal
conductivity.
[0166] In another aspect, the relationship between the first FWHM
and the second FWHM can be quantitatively calculated using the
value of Formula 1. The results shown in Tables 2 and 3 prove and
indicate that the thermal conductivity of the graphite sheet can be
predicted using the quantitative values for the relationship
between the FWHMs.
[0167] While the present disclosure has been described with
reference to the embodiments, it is to be understood that various
changes and modifications may be made without departing from the
spirit and scope of the present disclosure.
* * * * *